[settings] Re-add "uristring" setting type

[settings] Re-add "uristring" setting type

Commit 09b057c ("[settings] Remove "uristring" setting type") removed
support for URI-encoded settings via the "uristring" setting type, on
the basis that such encoding was no longer necessary to avoid problems
with the command line parser.

Other valid use cases for the "uristring" setting type do exist: for
example, a password containing a '/' character expanded via

  chain http://username:${password:uristring}@server.name/boot.php

Restore the existence of the "uristring" setting, avoiding the
potentially large stack allocations that were used in the old code
prior to commit 09b057c ("[settings] Remove "uristring" setting
type").

Requested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>